Facebook CEO Mark Zuckerberg has broken his days-long silence over a data scandal that has engulfed his company, admitting that the social networking site has “made mistakes”.

The scandal involves the personal data of up to 50 million users having been harvested and improperly shared with political consultancy firm Cambridge Analytica.

“We have a responsibility to protect your data, and if we can’t then we don’t deserve to serve you”, Mr Zuckerberg said in a Facebook post. “I’ve been working to understand exactly what happened and how to make sure this doesn’t happen again”.

It marked the second time in recent months Mr Zuckerberg has publicly pledged to “fix” the social media juggernaut – having said at the start of the year the company would do something about the exploitation by Russian-linked actors as part of Moscow’s efforts to meddle in the 2016 presidential election.

Zuckerberg has faced rising pressure to account for how Cambridge Analytica obtained data on millions of users, with a whistleblower alleging that the consulting firm put that information to use for Donald Trump’s presidential campaign in seeking to finely target and sway voters. Cambridge Analytica denies such information was used during the presidential campaign.

Facebook has said the fault lies with researcher Aleksandr Kogan – who created the psychological profiling quiz that gathered the data and who paid about 270,000 people to take it – for passing data to Cambridge Analytica. The information from millions of other profiles was said to be gathered from Facebook friends of those who took the quiz in the “This Is Your Digital Life” app. Last week, Facebook aid it had had asked both Mr Kogan and Cambridge Analytica to delete the information, but that it had received reports that not everything was removed.

Facebook suspended both parties from the social network and said that the passing on of information did not constitute a data breach – but that did little to quell what has become a mounting crisis.

With officials in Europe, America and the UK demanding to hear from Zuckerberg, the CEO has finally weighed in publicly, saying that the scandal was a “breach of trust” between the platform and its users.

Both Cambridge Analytica and Mr Kogan have sought to blame each other, with Mr Kogan telling BBC Radio 4’s Today programme that he was being used as a “scapegoat”.

“My view is that I’m being basically used as a scapegoat by both Facebook and Cambridge Analytica when… we thought we were doing something that was really normal,” he said.

In his post, Mr Zuckerberg said he learned of Cambridge Analytica’s alleged violations from reporters seeking to unravel the story. He said that Facebook became aware Mr Kogan had shared information with Cambridge Analytica in 2015.

“It is against our policies for developers to share data without people’s consent, so we immediately banned Kogan’s app from our platform, and demanded that Kogan and Cambridge Analytica formally certify that they had deleted all improperly acquired data. They provided these certifications”, Mr Zuckerberg said.

“This was a breach of trust between Kogan, Cambridge Analytica and Facebook. But it was also a breach of trust between Facebook and the people who share their data with us and expect us to protect it. We need to fix that”, Mr Zuckerberg said.

Chief Operating Officer Sheryl Sandberg, who has also faced criticism over her silence, said she deeply regretted “that we didn’t do enough” on Cambridge Analytica. “We’ve spent the past few days working to get a fuller picture,” she said.

Limits on data collection imposed since people downloaded Mr Kogan’s app would bar apps from “being able to access so much data today”, Mr Zuckerberg wrote, noting that apps can no longer draw in data from a person’s friends unless they have also consented.

Saying “there’s more we need to do”, Mr Zuckerberg pledged to investigate apps that were privy to large amounts of information before those new limits were imposed in 2014 and ban developers who refuse to be audited.

He also announced Facebook planned to restrict data access with new guidelines that include sealing off user data if people have not used an app in three months and limiting the data shared when a person signs in to her name, profile photo and email address.

The hotly anticipated statement came amid intensifying criticism and public pressure, with UK Prime Minister Theresa May questioning the company’s safeguards and a prominent technology executive urging people to abandon the site.

Calling allegations that Facebook user data was misused “very concerning”, Ms May told MPs in the House of Commons that “People need to have confidence in how their personal data is used.”

Ms May said she expected the company to “fully comply” with authorities investigating the controversy. Earlier this week, auditors retained by Facebook who were at Cambridge Analytica’s London offices stood down at the request of the UK Information Commissioner’s Office, which was seeking a warrant to examine the company’s data.

Members of Congress and Parliament have called on Mr Zuckerberg to testify in Washington and London. Paralleling the political fallout, signs also emerged of a consumer uprising goaded by players within the technology industry itself.

Billions has been wiped off Facebook’s stock value in recent days, amid fears that failure by big technology firms to protect personal data could lead to tighter regulation or deter advertisers. A founder of WhatsApp, a messaging service that was bought by Facebook in 2014, took to Twitter to urge his more than 30,000 followers to stop using Facebook.

“It is time”, Brian Acton wrote, appending a #deletefacebook hashtag that proliferated across the platform.

A shareholder lawsuit accused Facebook of harming investors by concealing information about the data transfer, noting that the company’s stock value has plunged, and a complaint filed by a user in Maryland accused Facebook of acting with “absolute disregard” for personal information and failing to halt “improper data aggregation”.

In response to the lawsuit, Facebook Deputy General Counsel Paul Grewal said in a statement that “we are committed to vigorously enforcing our policies to protect people’s information. We will take whatever steps are required to see that this happens”.